Many natural products which contain a chromophoric group capable of absorbing ultraviolet light undergo structural changes when exposed to sunlight. Such transformation are known either to bring about the formation of biologically active compounds or to bring about destruction of such materials, or both. Of particular interest in this present research are biologically active materials which contain a polyene chromophoric grouping since these light absorbing groups are related to the formation, transformation, or the destruction of vitamin D, retinal, and the leucotrienes. The project will be directed towards vitamin D itself and the hormonally-active derivatives of vitamin D. The role played by the triene unit in the biological activity will be evaluated by synthesis of vitamin D derivatives with the triene unit modified. Particular attention will be given to the pd-bonding and hydrogen bonding ability of the analogs as well as their polarity. Such studies will help to define the nature of the active site of the vitamin. A study of the photochemistry and photophysics in the whole vitamin D cascade will be studied in detail using narrow band light from tunable lasers and using solvent and temperature variations. This study should aid in the development of higher yields in photochemical reactions since detailed knowledge related to the excited state will permit better planning. As an alternative to the photochemical formation of the hormonally-active derivatives of vitamin D, simple, efficient convergent ground state synthetic procedures will be developed and will permit synthesis of the materials in quantity.